Clean up Makefile, tests and legacy code (#36638)

This simplifies the Makefile by removing the whole-file wrapping that
creates a tempdir introduced by
https://github.com/go-gitea/gitea/pull/11126. REPO_TEST_DIR is removed
as well.

Also clean up a lot of legacy code: unnecessary XSS test, incorrect test
env init, unused "_old_uid" hack, etc

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/hcaptcha/hcaptcha_test.go

@@ -8,7 +8,6 @@ import (
 	"io"
 	"net/http"
 	"net/url"
-	"os"
 	"strings"
 	"testing"
 	"time"
@@ -20,10 +19,6 @@ const (
 	dummyToken   = "10000000-aaaa-bbbb-cccc-000000000001"
 )
 
-func TestMain(m *testing.M) {
-	os.Exit(m.Run())
-}
-
 type mockTransport struct{}
 
 func (mockTransport) RoundTrip(req *http.Request) (*http.Response, error) {